Fluorosilicone rubbers have become important synthetic rubber products because of their temperature stability and high resistance to solvents such as jet fuel. The fluorosilicone gums which can be vulcanized to form fluorosilicone rubber compounds are typically fluoroalkyl-substituted diorganopolysiloxanes such as those disclosed in U.S. Pat. No. 4,029,629 (Jeram) and commonly assigned copending U.S. application Ser. No. 253,282, filed Apr. 9, 1981 and now abandoned.
The fluoroalkyl-substituted diorganopolysiloxanes are most advantageously formed by hydrolysis of cyclic polysiloxane monomers such as 1,3,5-tris(3,3,3-trifluoropropyl)-1,3,5-trimethylcyclotrisiloxane (or "cyclic trimer"); and the cyclic trimer is advantageously prepared from 3,3,3-trifluoropropene and halogenated silanes via reactions described by Pierce et al., Industrial and Engineering Chemistry, Vol. 52, No. 9 (Sept. 1960), pp. 783-4, and illustrated as follows: ##STR1##
3,3,3-trifluoropropene, therefore, can be seen to be an extremely important raw material in the production of fluorosilicone rubber. It is most commonly prepared by vapor-phase fluorination of halogenated hydrocarbons by exposure to hydrogen fluoride at elevated temperatures in the presence of a solid chromium oxyfluoride catalyst. Both the fluorination reaction and conventional chromium oxyfluoride catalysts are described in U.S. Pat. No. 2,745,886 (Ruh et al., 1956), U.S. Pat. No. 2,889,379 (Ruh et al., 1959) and Japan S.36 (1961)-16715 (Ruh et al., 1961), incorporated herein by reference.
Although the conventional chromium oxyfluoride catalysts widely used at the present time give extremely high reaction rates and yields initially, they are quickly deactivated by the formation of a carbonous deposit on the catalyst surface. The rate of deactivation is so rapid that economic operation of the fluorination process on an industrial scale is very difficult. All reported attempts to extend the life of chromium oxyfluoride catalysts by changing their composition or form have met with little success.
Extending the life of the catalysts by modifying process conditions has also been attempted, but these changes, while slightly prolonging catalyst life also result in decreased reaction rate, lower yield and disadvantageous process economics. For example, increasing the feed ratio of hydrogen fluoride to organic material slightly increases catalyst life but also increases the amount of unreacted hydrogen fluoride which must be recycled. Similarly, decreasing the reaction temperature tends to slow the deactivation of the catalyst but also decreases the reaction rate.
Various investigators and manufacturers of 3,3,3-trifluoropropene have noted that the presence of chlorinated organic compounds in the reaction can have a marked effect on the life of the chromium oxyfluoride catalyst. In particular, as described in Japan Kokai S.49 (1974)-133308 (Wada), the presence of either 1,1-dichloroethane or hexachloroethane in the reaction feed causes a lengthening in the catalyst life (defined by Wada as the time required for the yield to drop to 75 percent) of the chromium oxyfluoride. Several other chemically similar chlorinated or fluorinated compounds showed no such effect.
Using hexachloroethane with a chromium oxyfluoride catalyst results in a large improvement in catalyst life, but industrial use is hampered because of its physical properties. Hexachloroethane is a hard, semi-crystalline substance which sublimes at 187.degree. C. and which is essentially insoluble in hydrogen fluoride and only partially soluble in the halogenated hydrocarbons commonly used to produce 3,3,3-trifluoropropene, for example 1,1,1,3-tetrachloropropane. Since only a fraction of the hexachloroethane introduced to the reaction is converted to a volatile product, the remaining hexachloroethane precipitates (in the absence of any material in which it is soluble) as the gases exiting the reaction chamber are cooled, causing clogging of processing lines and other difficulties.
Consequently, there is a need for a catalyst composition which will promote vapor-phase fluorination of halohydrocarbons, which has a long catalyst life, but which will be readily adaptable to industrial scale processing.
All of the patents and the application mentioned above are hereby incorporated by reference.
It has now been discovered that significant improvements in the life of chromium oxyfluoride catalysts can be achieved by a novel method for their preparation and/or employing, as part of the catalyst composition, chlorine or pentachloroethane to activate the chromium oxyfluoride.